Add regression coverage for runtime data-mode service

## Problem
The current test suite does not cover runtime data-mode service.

## Expected behavior
- A focused Vitest case locks the existing behavior.
- The targeted test file passes and fails if the covered behavior regresses.

## Scope
Only add regression coverage for $(@{Slug=data-mode; Focus=runtime data-mode service; PrTitle=test(data-mode): cover the runtime data-mode service; TestFile=web/src/services/__tests__/dataMode.test.ts; Mode=new; Append=/*
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import { afterEach, describe, expect, it } from 'vitest';
import { useDataModeStore } from '../../stores/dataModeStore';
import { isMockMode } from '../dataMode';

describe('dataMode service', () => {
afterEach(() => {
useDataModeStore.setState({ useMock: false });
localStorage.clear();
});

it('reflects the runtime mock-data store flag', () => {
useDataModeStore.setState({ useMock: true });
expect(isMockMode()).toBe(true);

useDataModeStore.setState({ useMock: false });
expect(isMockMode()).toBe(false);
});
});}.TestFile). No runtime behavior changes.

## Acceptance criteria
- [ ] The targeted Vitest file passes.
- [ ] git diff --check is clean.

Start with web/src/services/dataMode and web/src/stores/dataModeStore, then add the focused test at web/src/services/__tests__/dataMode.test.ts. Run the targeted Vitest file; done means the runtime data-mode behavior is covered, the test passes, and git diff --check is clean.

Written by the indexing model from the issue text.